]26 



CCRLJIELMIKriIES. 



other dorsal, tlieir free edges bearing bristles. Each mantle 

 secretes a shell largely composed of carbonate and phosiDhare of 

 Inne. In a few the dorsal and ventral shells are similar, but 

 usually the ventral valve (in Crania attached directly without the 



Fig. 299. — WnldlieimiaftdveRcena. (From Zittel.) Shell "^ith arms and muscles. /7, arm 

 witli fringed border Ot): c, c\ divaricators; d, adductors; A hinge process ithe 

 vertical line shows position of hinge). 



intervention of a stalk) is more strongly arched and has an opening 

 at the posterior end for the jiassage of the stalk (tigs. 299, yOO). 

 The flatter dorsal valve frequently bears a characteristic feature in 

 the skeleton of the arms (fig. 300) which, when present, has greatly 



Fig. 'M).— Waldheimia flavesceus. (From Zittel.) .t, dorsal, B, ventral valve; n, h. c, 

 impressions of muscular insertions; n. adductors; h", ad.itistors (stalk muscles); 

 i; (■', divaricators; .■.-, hinge groove of upper valve in which the tooth it) of the 

 lower valve passes ; /, support of arms; i(, deltidium; /, foramen for stalls. 



different exjiression. Its basis consists of two calcareous rods 

 which, bilaterally symmetrical, project downwards from the dorsal 

 valve. These may l)c connected by a curved transverse band, and 

 from their ends a spiral process may extend on either side. This 

 ap])aratus supports the spiral arms. When closed the valves com- 

 pletely enclose the body. When they open the gape is anterior, 



